A photochemical reaction indicates the whole of chemical reactions each in which molecules are brought into a state having a high energy level, a so-called excited state by photoirradiation, namely by making a radical reaction initiator absorb energy ascribed to the irradiated light, and the reaction is caused by the excited molecules. The photochemical reaction includes kinds of oxidation-reduction reaction by light, substitution-addition reaction by light, etc., and it is known that the applications include photo industry, copying technology, induction of photovoltaic power, and in addition, synthesis of organic compounds. Further, as unintentional photochemical reaction, photochemical smog and the like also belong to photochemical reaction.
For example, it is known that cyclohexanone oxime can be synthesized by photochemical reaction, and photonitrosation of cycloalkane is also a widely known technology at the present time.
As light sources for the photochemical reaction which have been used so far, in most cases, a lamp, in which mercury, thallium, sodium or another metal is enclosed in an environment of vacuum or close to vacuum, voltage is applied, and the emitted electron beam is irradiated to the enclosed metal, and the light emitting ascribed to electric discharge in gas or vapor condition is utilized, for example, a discharge lamp or a fluorescent lamp, is used as the light source.
For example, in case where a high pressure mercury lamp is used as a light source, the effective wavelength is 365 nm to 600 nm. However, in this type of discharge lamp using mercury, specific light emission energy due to mercury exists also in the wavelength region including ultraviolet rays of less than 365 nm. Therefore, for example, in case of having light emission energy in a short wavelength region including ultraviolet rays of less than 350 nm, because it is comparable to the dissociation energy of many chemical bonds, a reaction other than the purpose proceeds and promotes a side reaction, and a brown tar-like deposit is formed on the photoirradiation surface of the discharge lamp, thereby reducing the yield. Therefore, in order to cut the ultraviolet rays, a water-soluble fluorescent agent or a UV-cut glass is used.
In order to reduce such problems in a mercury lamp and improve the luminous efficiency, it is known that a thallium lamp exhibiting light emission energy effective to a wavelength of 535 nm and a sodium lamp exhibiting light emission energy effective to a wavelength of 589 nm are effective. By using a sodium lamp as a light source, the yield is dramatically increased and a stable reaction becomes possible. Further, by using a high-pressure sodium discharge lamp, the industrially effective wavelength is set at 400 to 700 nm, and the efficiency can be increased in the wavelength region of 600 nm to 700 nm. The peak wavelength in this range can be estimated to be about 580 to 610 nm. However, in order to improve the electric properties and starting of the discharge lamp, coexistence of mercury is inevitable, and a filter for cutting ultraviolet rays due to mercury is necessary. In particular, short wavelengths less than 400 nm generated by mercury are unnecessary wavelengths because they have excessive energy and cause unnecessary side reactions.
Furthermore, the sodium lamp has a peculiar light emission energy peak in a wavelength region including infrared rays having a wavelength of 780 to 840 nm, and its energy intensity is frequently comparable to the maximum light emission energy of the sodium lamp. Since the dissociation energy of nitrosyl chloride is about 156 J/mol, which is comparable to the light emission energy at a wavelength near 760 nm according to Einstein's law, the light energy is small in the longer wavelength region and the nitrosyl chloride does not dissociate, and therefore, it does not contribute to a reaction and causes a great energy loss.
On the other hand, light emitting diodes, also abbreviated as LEDs, have the advantage capable of converting electrical energy directly into light using semiconductors, and are attracting attention in terms of suppression of heat generation, energy saving, long life, and the like. Its history of development is still shallow, red LEDs were commercialized in 1962, LEDs such as blue, green and white were developed from around 2000, and they were commercialized for display and lighting uses. On the other hand, a discharge lamp used for a photochemical reaction has a very high output and a high luminous efficiency, but if it is attempted to obtain the light emission energy required for a photochemical reaction equivalent to that of a discharge lamp by LEDs, the required number of LEDs becomes enormous, and it has been considered that it is difficult to apply LEDs as a light source for a photochemical reaction, because problems in circuit design, LED heat countermeasure and cost remain. Furthermore, it is necessary to irradiate a reaction liquid with uniform light for the photochemical reaction, but the LED has a strong directivity and it is difficult to obtain the wavelength necessary for the reaction with a high efficiency, and also from this point of view, application of LEDs to the light source of the photochemical reaction has been considered to be difficult.
However, recently, as described in Patent document 1, there is an example in which photochemical reaction by LEDs is carried out using a small reaction apparatus, and moreover, as described in Patent documents 2 to 4, solution of the problems for enlarging a light-emitting body is being in sight.